In Situ Construction Of Silica Nanofiber-based Aerogels At Room Temperature And Their Mechanical Properties

Ceramic aerogels are attractive for a wide range of applications in the fields of aviation,defense industry,environmental governance,and biomedicine owing to their integrated properties of high porosity,ultralow density,high specific surface area,low thermal conductivity,chemical stability,and thermal stability.At present,ceramic aerogels are mainly prepared by the sol-gel method,and the bead chain-like three-dimensional skeletal structure resulted in their brittleness and low strength.In order to improve the mechanical properties of aerogels,researchers introduced fiber-reinforced materials into aerogels to enhance their skeletal structure by mixed gel method,sol casting,aerogel powder blending,etc.The fiber-reinforced ceramic aerogels have certain mechanical strength while maintaining its own properties.However,the fibers and aerogel powder are only connected by intermolecular forces,so the composite aerogel still suffer from poor vibration resistance and unstable structure in actual use,which limits its application in potential load-bearing fields.Researchers have fabricated ceramic fibrous aerogels through direct spinning,layer-by-layer stacking,atomic layer deposition,and other methods,but the aerogels prepared by these methods generally have poor mechanical properties and high density,and an unavoidable calcination are needed to remove the organic components,which cannot satisfy the requirements of practical applications.Most recently,the ceramic nanofibrous aerogels with adjustable microstructures,controllable shapes and sizes,and low density have been fabricated by freeze-drying method.The resulting ceramic materials exhibited good elastic recovery,but this method still requires subsequent high-temperature calcination to construct stable bonding structure among fibers,which makes it difficult to further improve the mechanical properties.Therefore,the construction of ceramic fibrous aerogels with stable structure,adjustable bulk density and microstructures,and excellent mechanical properties has important theoretical significance and application value.In this paper,we have carried out a series of research work about in situ construction of SiO₂ nanofiber-based aerogels at room temperature and their mechanical properties.The superelastic SiO₂ nanofiber-based aerogels have been in situ constructed at room temperature by combining flexible electrospun SiO₂ nanofibers and hydrolyzed silane sol.The dynamic crosslinking process of single and binary silane sol was investigated at the molecular level,and the structural characteristics of the crosslinking network were studied for the optimization of the mechanical properties of the aerogels.Moreover,we have prepared a superelastic SiO₂nanofibrous dual-network aerogel by introducing the secondary fine network structure of bacterial cellulose.The mechanical properties of the aerogels have been further improved by using the“multilevel network synergistic crosslinking”method.The main findings are summarized as follows:（1）By investigating the preparation parameters of SiO₂ nanofiber dispersion and freeze-shaping process,a homogeneous and stable nanofiber dispersion and a well-shaped frozen block were obtained.The hydrolyzed silane sol could form stable Si-O-Si bonding network among fibers during the freeze-drying process,and the superelastic SiO₂nanofibrous aerogels with a biomimetic Juncus pith-like framework structure were constructed in situ at room temperature.The multilevel compressive deformation mechanism of aerogels was proposed,and the molecular dynamics simulations were used to study the structural characteristics of the three kinds of crosslinking network of silane sol binders.It was found that the SiO₂ nanofibrous aerogels with methyltrimethoxysilane as the binder exhibited the optimal mechanical properties.Subsequently,we studied the compression resilience of aerogels.The maximum stress was 19.2 k Pa at the compressive strain of 80%,and the plastic deformation is only 1%after 1000 cyclic compression experiments（compressive strain of 50%）,which is better than that of the existing ceramic fibrous aerogels.The temperature resistance of aerogels was further investigated.The storage modulus and loss modulus remained nearly identical after being compressed for one million cycles at an oscillatory strain of5%at 500^oC,which is comparable to that of the previously reported carbon aerogels（one million cycles at 2%strain）.In addition,the aerogel could recover to its original shape rapidly from compression in butane blowtorch flame（outer flame temperature as high as 1100^oC）in air or in liquid nitrogen（-196^oC）.The excellent temperature resistance and low thermal conductivity of the aerogel make it promising candidate for the next-generation thermal insulation materials in extreme environments.（2）We have fabricated a superelastic SiO₂ nanofiber/nanoparticle aerogel by the integration of SiO₂ nanofibers,SiO₂ nanoparticles,and the binary silane sol of ethyl orthosilicate and 5,5-dimethyl-3-（3’-triethoxysilylpropyl）hydantoin.The aerogels exhibited a loofah-like skeletal structure with a functional Si-O-Si bonding network.The structural characteristics of the crosslinking network formed by the binary silane sol were investigated,and the“stiff-soft binary synergistic”bonding mechanism was proposed.Subsequently,the mechanical properties of the aerogel were studied,and it was found that the maximum compressive stress of the aerogel under 80%compressive strain was 30.7 k Pa,which was higher than that of the SiO₂ nanofibrous aerogel crosslinked by a single silane sol and 2～6 times higher than that of other ceramic or carbon aerogel materials.The as-prepared aerogel exhibited not only superelastic in air but also excellent elastic recovery in water.After being compressed 100 cycles in water,the plastic deformation of the aerogel was only 7%,showing underwater superelasticity.Combining the superhydrophilicity,excellent mechanical properties,and the N-halamine bonding network,the aerogels showed great potentials in the field of antifouling water disinfection.While ensuring high bactericidal efficiency,the water filtration flux of the aerogel could reach up to 57600 L m^-2 h^-1,which was 1～2 orders of magnitude higher than that of other filter materials.（3）Based on the research work of silane crosslinking network,we further introduced bacterial cellulose（BC）nanofibers into the dispersion of SiO₂ nanofibers and silane sol to prepare a superelastic SiO₂ dual-network structured nanofibrous aerogel.The SiO₂ nanofibers entangled with each other formed a macroporous fibrous skeleton with a pore size of about 20μm,and the BC nanofibers with a smaller diameter further formed a secondary fine network structure with a pore diameter of about 200nm on the skeleton.Combining the freeze-shaping process of the dispersion and Gaussian calculation,we analyzed the formation mechanism of the dual-network structure.Subsequently,we studied the mechanical properties of aerogels.The synergistic crosslinking effect of the multilevel nanofibrous network of aerogels endowed it with superelastic properties.The maximum compressive stress was 83.3k Pa under 80%of compressive strain,which is much higher than that of single silane sol and dual silane sol crosslinked aerogels.Moreover,the aerogel exhibited foldable properties,the storage modulus and loss modulus remained stable after 100,000 fatigue buckling with buckling strain of 100%,showing excellent structural stability.In addition,we also explored the application of aerogel in the field of antibacterial and antiviral air purification,the aerogel could inactivate 6 log E.coli and bacteriophage,exhibiting great potentials in the fields of air purification.